The present invention relates to seals for the pressure cushion of a laminating machine operating according to the pressure cushion principle.
Machines such as double belt presses perform laminating and compression procedures in which a strip of material to be compressed is fed between two traveling metal belts which press against opposite surfaces of the material in order to compress it. Such presses are composed essentially of two metal belts each mounted on a respective support structure and between which a compression region is formed. The belts are advanced around their respective support structures so as to convey the material being worked upon through the press as that material is being pressed.
One known type of double belt press is constructed to operate according to the pressure cushion principle. For this purpose, the support structure associated with each belt is provided with a pressure plate located behind the belt in the compression region and provided with a slide surface seal forming a frame which encloses a pressure cushion formed by a liquid of gaseous pressure medium. The major faces of the cushion are bounded by the surface of the pressure plate and the rear surface of the associated metal belt, and the slide surface seal cooperates with the pressure plate and the rear belt surface in order to seal the region containing the pressure medium. The pressure medium acts against the rear surface of the associated metal belt to press it against the work material.
Due to the frictional engagement between the metal belt, which is generally of steel, and the slide surface seal, the latter is subjected to a substantial degree of wear and thermal stresses. As a result, the strips forming such seals can be manufactured of only a few existing materials which are all relatively hard and unyielding. However, because of inherent limitations in the processes employed for manufacturing the steel belts, and the operating conditions to which those belts are subjected, the rear side of such a belt which contacts the slide surface seal cannot maintain a perfectly planar, smooth surface configuration where it contacts the seal. Therefore, in view of the relatively hard nature of known seal materials, there is a tendency for more than an acceptable amount of pressure medium to be lost from the pressure cushion by leakage past the seal thereof, and this will adversely influence the performance of the laminating machine and its economy of operation.
The use of a liquid pressure medium for the pressure cushion, while it is inherently desirable, becomes unacceptable when even small quantities of fluid are able to leak past the seal and accumulate in a region outside of the pressure cushion, although a minimum amount of leakage would be advantageous for lubrication purposes to increase the service life of the sealing strips.
Typical of the state of the art in this area are the disclosures of German Patent No. 2,722,197 and U.S. Pat. No. 2,135,763 which present, however, no detailed teachings regarding the configurations of the sealing strips, or packings, disclosed therein. In particular, these reference provide no indication as to how to deal with the unavoidable wear experienced by such sealing strips or the resulting formation of gaps during operation of the machine. Generally, the bearing and gap equilization function performed by slide surface seals cannot be taken over by elastic sealing components of the type customarily used in other areas of the hydraulic art without also introducing special measures to minimize gap formation.